I have a small terraced house with an extension. the main house has a modern consumer unit with RCD fed from the meter tails and MCB's. The extension which has a kitchen and bathroom has its own modern CU with RCD and MCB's. This CU is fed from the main CU via 16mm cable from an MCB in the main CU. The cable length is about 7metre long under uninsulated floor.

Wired to extension CU is: 5 socket ring main, lighting, underfloor heating (less than a KW), 3.7KW electric oven,combi boiler and one spur for Dishwasher and Washing machine (never used together).Question is should the extension CU be supplied directly from the mains via a splitter box rather than from the an MCB in themain CU. Billobach

In the main no, having it supplied from the largest MCB the main CU will take is the better method.

There are exceptions but to start with the link cable would need to be of a type not needing RCD protection and of a size able to take the full load the DNO fuse will allow, this often means 25mm rather than 16mm.

Thanks ericmark.The other suggestion I have had is to install an RCD Main switch between the new 16mm tails and the 16mm cable to the kitchen CU.Also the main electricity board fuse is 60amp. SWALEC the supplier said that because it's a small terraced house there is no need to have a bigger main fuse.

Depends what type the 16mm cable is. The RCD rules cover buried cable without protection there is a list of cables which don't need RCD protection BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436, do not require RCD protection so that's SWA and Ali-tube oddly the steel braided cable is not in the list. If I don't need to protect the cable then I would not, that way I can use RCBO's in kitchen or a master RCD in kitchen so the RCD is local and easy to reset. The more the RCD covers the more likely to trip without good cause. Surface cable does not need RCD protection.

As to 60A DNO fuse not really a problem my house is likely a 60A no electric shower so biggest power user in the cooker. House originally had a 4 way fuse box with 2 x 32A, 1 x 16A and 1 x 5A fuse so unlikely to exceed 60A for long enough for it to blow. However 60A fuse is in a 100A fuse holder and should it blow as long as the tails are big enough the DNO would replace with a larger fuse. Therefore good policy to ensure tails will take 100A even if the fuse is only 60A. If and when a fuse blows you don't want to have to renew tails as an emergency to get larger fuse.

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